This invention relates to a mechanical filter device having a damper for protecting a tuning fork from outer impact, shock and mechanical noises. In conventional filter devices, it is necessary to satisfy the following requirements:
1. The mechanical filter should not undergo mis-action caused by mechanical noise during carrying,
2. The mechanical filter should not change its vibrating characteristics and should not break when subjected to a big outer impact.
In one conventional type, there is provided a soft supporting construction for the tuning fork of the mechanical filter and in another type, the mechanical filter has masses attached to the tuning fork at its leg portions for satisfying the above requirements.
However, in the former type mechanical filter, when the mechanical filter receives a strong outer impact such causes a permanent change of the vibrating characteristics where upon the filter becomes of inferior quality.
Further in the latter type mechanical filter, the head portion of the tuning fork becomes heavy and the anti-impact characteristics change for the worse.
Fig. 1 shows a cross sectional view of the conventional mechanical filter which comprises a terminal plate 2 mounted to a lower opened portion of a casing 1 whereby said casing 1 is sealed. A supporting rubber member 3 for supporting a tuning fork is positioned in a central recess portion of said terminal plate 2, the terminal pins 4 are respectively inserted and mounted on both side portions of said terminal plate 2. A U-shaped tuning fork 5 having vibratable tines is supported by said supporting rubber by embedding a stem portion 5a of the tuning fork into the supporting rubber member 3.
A weight member or mass 6 is attached to the end portion of each tuning fork tine and a piezoelectric element 7 is mounted on the lower side portions of the tuning fork tines and are electrically connected to the terminals 4 by lead wires.
The conventional mechanical filter is 7a and 7c, the lead wire 7b connecting the stem vibrates at a peculiar or spurious vibrating frequency when subjected to mechanical noise and even though the tuning fork 5 is softly supported by the supporting rubber member 3, such a said construction does not adequately protect the tuning fork against outer impact.
Further, the tuning fork is relatively heavy at its vibratable leg portions due to the masses 6 and therefore is likely to strike the inner wall of the casing and become damaged by the outer impact whereby there are many problems such as of breaking, falling of the peculiar vibrating frequency and increasing of the insertion loss. In order to eliminate the above drawback and insufficiency, the conventional mechanical filter was developed as indicated in Fig. 2, wherein a damper 8 is attached to both sides of the inner portion of said casing.
However, in devices having a small casing, the attaching of said damper to said casing is extremely cumbersome and takes a long time whereby it is not preferable to satisfy production requirements. Further when said mechanical filter having such a small casing undergoes the temperature test there is a good possibility of loosening the attached rubber member.
Fig. 3 shows the construction of attaching a frothy rubber member 9 to the leg portions of said tuning fork instead of said weight member 6, however, it is not preferable to use such a rubbery type material as it possesses a hygroscopic character and therefore greatly influences the particular vibrating frequency and hence is undesirable from the standpoint of stability of vibrational frequency.